**4.3 Conclusions**

Raw, unsterilized UF whey permeate could be a substrate for biohydrogen production. Exploitation of UF whey permeate as a feedstock for H2 and biogas production is an attractive and effective way of wastewater treatment with simultaneous renewable energy producing. Several factors influenced H2 production in continuous bioreactors, especially pH of operation, HRT and OLR. The experiment showed, that pH 5.8 in the hydrogenogenic phase was not sufficient to inhibit the activity of methanogenic bacteria. Lower HRT values (<24 h) should be apply to eliminate methanogenic bacteria. Moreover, the regulation of pH in the hydrogenogenic reactor should be coupled with the influent pH adjustment in order to avoid lactic acid production in the raw wastewater. The HRT of 12 – 24 h and OLR of 10 – 20 g COD L-1 d-1 were found to be sufficient for effective H2 yield.

This study demonstrated that biohydrogen production from UF whey permeate can be efficiently coupled with methane production in a subsequent step. For hydrogen production stage in stable conditions (HRT of 12 h, OLR of 20 g COD L-1 d-1, pH 5.2) hydrogen production rate was 0.97 L H2 d-1 and hydrogen content in the biogas reached 29,8% v/v. In the methanogenic step, the average biogas and methane production rates were 3.0 L d-1 and 2.2 L CH4 d-1, respectively, while the methane content in biogas approached 71% v/v. The two-stage fermentation system reached 95% of COD removal efficiency.

The results of these investigations made the base for further studies to find the optimal operating conditions for higher biohydrogen production in UASB reactor in two-stage fermentation process with respect to determine the optimal OLR, HRT and pH parameters of the reactor giving stable long-term generation of H2 from raw UF whey permeate.
